1. Field
The aspects of the disclosed embodiments relate to a primary structure for aircraft of composite material with improved crash resistance and an associated energy-absorbing structural element. The specific purpose of the disclosed embodiments is to absorb the energy dissipated from a violent compressive force, in particular from a shock following an accident such as a violent landing or water landing stressing the fuselage along its vertical axis (crash with vertical component).
2. Brief Description of Related Developments
The fuselage of an aircraft includes principally a structure composed of a skin reinforced on the inside by reinforcing frames and rails. The reinforcing frames are positioned on cross sections of the fuselage essentially perpendicular to a longitudinal axis of the fuselage, while the rails extend essentially along the longitudinal axis. The reinforcing frames support crossbars that are generally straight and horizontal, in an aircraft frame of reference, to which are fastened the flooring panels.
At the time of a crash with a principal vertical component, the lower part of the fuselage is generally the first area of the aircraft subject to impacts, and it accordingly participates in the absorption of the energy from these impacts.
The certification of aircraft, in particular civil transport airplanes, imposes behavioral criteria on the fuselage in different crash situations to improve the chances of passenger survival.
For aircraft that have a fuselage structure made from structural elements (reinforcing frame, skin, rails, crossbars) of metallic material, a large amount of the energy of impact is absorbed by the plastic deformation of the metallic structural elements. Accordingly, it is generally unnecessary to provide dedicated energy-absorbing systems, since the structure passively assures the function of energy absorption because of the intrinsic properties of the metallic alloys used and a suitable design.
On the other hand, the fuselage structures made from elements of composite material have practically no plastic region before breaking, contrary to metallic structural elements. A fuselage structure of composite material accordingly behaves very differently from a fuselage structure of metallic material, with regard to the absorption of energy. Actually, after stress, either before or after breaking, the structure of composite material restores a large portion of the energy absorbed at the time of the elastic deformation.
To permit the composite structure to absorb the energy generated by the crash, a known method described in the French Application with the File Number 0850966 consists of utilizing energy-absorbing elements installed along a direction of compression of the forces to be absorbed, between the reinforcing frames and the crossbeams fastened to these frames. Each of these absorbing elements has a compression beam and a bracket, so that when the compression beam is subjected to a compressive force corresponding to the forces generated by the crash, it is pressed against the bracket provided with cutting elements that destroy the beam, so that the progressive destruction of the compression beam absorbs part of the energy of the impact.
This method is satisfactory but nevertheless rather onerous since it requires the production of appropriate energy-absorbing elements: compression beams and brackets, since they are not available as such on the market, or not readily obtainable from existing elements. It is also possible to encounter difficulties in assembly for positioning the compression beam relative to the bracket.
Accordingly, there is a need for an equivalent low-cost solution that is easy to install.